This invention relates to an improved composition for inhibiting deposition of scale and sludge in aqueous systems. More specifically, small quantities of a composition consisting of sodium carboxymethylcellulose ("CMC") and either gluconic acid or glucoheptonic acid or both, or their water soluble salts are added to the steam generating systems to prevent the formation of scale and sludge deposits therein.
The formation of scale and sludge deposits on heating surfaces is the most serious problem encountered in boiler operations and is the result of entrained scale-forming impurities, such as calcium and magnesium salts, and iron. External softening of boiler feedwater partially removes these substances, but internal chemical treatment is typically applied in industrial boilers and other aqueous systems in order to counteract scale formation. The chemical treatment of boiler water generally involves the combined use of a precipitating agent and a sludge conditioner to maintain the boiler sludge in a fluid form. The chemicals most commonly employed for precipitation of calcium salts are soda ash and the sodium phosphates, for example disodium phosphate, trisodium phosphate and sodium metaphosphate. Magnesium salts are precipitated as magnesium hydroxide by the alkalinity of the boiler water.
The precipitate, or sludge, is rendered non-sticky and easily manageable by the addition of a sludge conditioner, and it is removed from the boiler periodically through the blowdown. Compounds such as polyacrylates, polymethacrylates and carboxymethylcellulose have been used as sludge conditioners in certain settings, such as in food processing plants and in United States Department of Agriculture (USDA) inspected meat and poultry plants. However, in many food processing plants it is required to use only those boiler water additives which are generally recognized as safe ("GRAS") by the Food and Drug Administration ("FDA"). For example, GRAS substances are often preferred where the steam produced in the boiler or steam generating system will contact the food being processed. Polyacrylates and polymethacrylates are not GRAS substances.
Sodium-CMC is a GRAS substance and previously has been used for sludge conditioning, either alone or in combination with other substances. For example, U.S. Pat. No. 3,188,289 (Kahler et al.) discloses the reduction or prevention of deposits in steam generating systems by introducing the sodium salt of CMC into the feedwater. U.S. Pat. No. 3,483,033 (Casey) discloses minimizing the formation of scale deposits during evaporation of raw cane and beet sugar juices by adding a composition comprising a hydrolyzed polyacrylamide, a protective colloid such as sodium alginate or CMC and preferably one or more other ingredients selected from the following: EDTA, a gluconate such as for example sodium-gluconate or sodium gluco-heptonate, and polyphosphates. U.S. Pat. No. 4,085,060 (Vassileff) discloses a composition capable of sequestering metal ions which is a mixture of at least two different hydrophilic polymers, one of which may be CMC with a molecular weight between 600 and 60,000.
Gluconic and/or glucoheptonic acid have been used as sequestering or chelating agents. U.S. Pat. No. 3,308,062 (Gunther) discloses the use of gluconic acid in steam boilers, evaporators, stills and the like in an amount to prevent the formation of scale. U.S. Pat. No. 3,328,304 (Globus) discloses sequestering and/or chelating agents useful in the dairy, brewery and pharmaceutical industries which are prepared by admixture of citric acid (anhydrous), D-gluconic acid (primarily as the lactone), magnesium hydrocarbonate, magnesium acid citrate, and water both in the combined and free states.
Experience has shown that sodium-CMC alone is not completely effective in preventing the formation of boiler scale. Indeed, the compositions discussed above comprise both CMC and one or more other compounds, not all of which are GRAS substances. The composition described herein is an improved sludge conditioner in which the sludge conditioning capabilities of CMC alone have been improved by the inclusion of gluconic acid or glucoheptonic acid or their water soluble salts.